1. Field of the Invention
The present invention relates to a communication system transmitting an encoded signal across a channel with noise and fading.
2. Description of the Related Art
A communication system includes a transmitter and a receiver connected by a communication channel. In a wireless communication system, especially a mobile wireless communication system, the condition of the communication channel can change rapidly. The capacity of the channel varies with the channel condition. That is, the channel can hold more information when channel conditions are good, and less information when conditions are bad. If the transmitter and receiver are designed with parameters fixed to levels that assume less than optimal channel conditions, then they will not use the full capacity of the communication channel when the channel conditions approach the optimum channel conditions.
For example, radio waves propagate between the transmitter and the receiver in a manner that causes level of the signal received by the receiver to fluctuate constantly. This is known as fading. The signal-to-noise ratio (SNR) of the channel fluctuates in accordance with fading. The capacity of the channel fluctuates with variation in the SNR. That is, whenever the SNR improves, the capacity of the channel increases, and whenever the SNR decreases, the capacity of the channel decreases.
In order to best make use of the capacity of the channel, the information rate, that is, the rate of information transfer, should match the channel capacity. In a fixed information rate system, the information rate is set assuming poor channel conditions so as not to risk losing information. As a result, some of the full capacity of the channel will be wasted whenever fading condition improve to better than the assumed poor channel conditions.
Adaptive communication systems have been proposed that instantaneously monitor the channel conditions, and change various transmission parameters depending on the present channel state in order to maximize efficiency of transmission.
For example, U.S. Pat. No. 5,907,563, based on foreign priority to Japanese Patent Application No. 8-078157, discloses a wireless data communication apparatus that changes error control method based on the channel conditions. That is, during data communication, the apparatus observes various condition parameters, such as fading period, delay variance, and propagation level ratio of line-of-sight and trans horizon, to judge the condition of the wireless transmission channel. The apparatus then selects an error control strategy, such as protocol, block length, and parity length, according to the channel condition.
For example, when the system detects transmission channel conditions of fading period: 20, delay variance: 0.5, and propagation level ratio: 100, then the system will select an error control strategy of Bose Chaudhuri Hocquenghem (BCH) code as the protocol, a block length of 100 symbols, and a parity length of 10 symbols. On the other hand, when detected transmission channel conditions include a fading period of 50, a delay variance of 0.1, and a propagation level ratio of 0, then the selected error control strategy includes Solomon Reed (SR) code as the protocol, a block length of 150 symbols, and a parity length of 30 symbols.